1. Field of the Invention
The present invention relates to an image processing device for determining an image area in which character, photo, and dot images are mixed, and, more particularly, to an improvement in the device for accurately determining whether an image is a character image or another type of image.
1. Description of the Prior Art
In general, a facsimile device or a phototelegraphic device uses an image processing device for generating a pictorial signal by scanning a document in which photo images, character images, and dot images are mixed, converting it to binary signal 0 or 1, and coding them. However, in the process of binary signal conversion, if the image is converted to binary signal 0 or 1 in exactly the same way regardless of the type of image, the coding efficiency drops and image reproducibility deteriorates. Thus, the conventional image processing device solves such problem by determining the type of image, and generating binary signals with methods suitable for the respective images.
In relation to the image processing technique for determining the type of image, U.S. Pat. No. 4,547,811 discloses a method for distinguishing a binary image from an image with gray levels, such as a photograph. The method performs the following determination by dividing a document into blocks each composed of m x n picture elements or pixels, detecting the maximum density level L.sub.max and the minimum density level L.sub.min of picture signal levels in the blocks, and comparing them with a reference value P.
i) The image in a block is a character image if EQU L.sub.max -L.sub.min &gt;P, PA0 ii) The image in a block is a photo image if EQU L.sub.max -L.sub.min .ltoreq.P.
and
However, while the above method is effective in distinguishing a photo image whose density level continuously varies and a character image, it is not effective in distinguishing a dot image like a mesh pattern and a character image.
As an alternative image processing technique, there is a method which utilizes the characteristic that a character image has a larger differential amount or density difference at the boundary, and determines an image to be a character image if the differential amount is large, and not to be a character image in other cases. However, because the differential amount is large at a boundary not only of a character but also of a dot in a dot image, there is a higher possibility for a dot image to be determined to be a character image.
If a dot image is determined to be a character image and its picture signal is converted to binary signals in a binary conversion rule of suitable for the character image, the reproducibility of the dot image deteriorates, and the reproduced image becomes rather dark and obscure.
In addition, U.S. application Ser. No. 08/260,798 filed by the applicant on Jun. 16, 1994 proposes a device for image processing which can accurately identify areas of a dot image and a character image.
In general, an image mainly consisting of line drawings, such as a character image, not only has a distinct boundary between low and high densities but also is composed of continuous edges in which the boundary length continues for two or more picture elements in the same direction. However, an image consisting of dot drawings, such as a dot image, has edges but is rarely composed of continuous edges continuing for two or more picture elements in the same direction. The device for image processing device disclosed in the above application identifies dot images and character images by detecting such continuous edges.
More specifically, a detection circuit for detecting a continuous edge detects a continuous edge, in which a boundary with a large density difference continues for two or more picture elements, from picture signals of picture elements in a primary scanning direction and a secondary scanning direction in an image. The detection circuit for detecting a continuous edge generates a first detection signal when it detects the continuous edge from picture signals of picture elements in the primary scanning direction, and generates a second detection signal when it detects the continuous edge from picture signals of picture elements in the secondary scanning direction. At the generation of at least one of the first and second detection signals, the picture signals are converted into binary signals as a character image. At the generation of neither the first nor second detection signal, the picture signals are converted into binary signals by binary conversion different from that of the first conversion circuit.